Inkjet printers have conventionally been used to print an image by ejecting fine droplets of ink from a plurality of ejection ports of a head unit toward printing paper while moving the printing paper relative to the head unit. In recent years, inkjet printers have also been employed to print label stickers. Since label stickers are printed on base members that are not easily impregnated with ink, such as sheets of resin including polyethylene terephthalate (PET) or coated paper, the printing of label stickers uses, for example, ultraviolet (UV) curing ink and cures (fixes) the ink deposited on the base member by irradiation with ultraviolet rays.
Japanese Patent Application Laid-Open No. 2011-79952 (Document 1) discloses an ink set in which, in a surface lifetime region of less than 100 ms after ink landing, the dynamic surface tension of each color ink is higher than or equal to that of a black ink; in a surface lifetime region of 100 to 3000 ms after ink landing, there is a point at which the dynamic surface tension of the black ink is equal to that of each color ink; and in a surface lifetime region at and subsequent to that point, the dynamic surface tension of the black ink is higher than that of each color ink. The ink set disclosed in Document 1 can reduce bleeding on not only plain paper but also paper with poor ink absorbency.
Incidentally, in the case of ejecting a plurality of color inks onto a base member, a first ink ejection part that ejects a first ink of one color and a second ink ejection part that ejects a second ink of another color are arranged in the direction in which the base member is conveyed relative to a head unit. Thus, droplets of the first ink are deposited (lands) on the base member, and then droplets of the second ink are deposited on positions adjacent to the droplets of the first ink after a predetermined period of time has elapsed since the landing of the droplets of the first ink. The method of Document 1, however, fails to take such a time difference into consideration and thus cannot reduce bleeding at a boundary between areas of the first and second inks on the base member or variation in the position of the boundary.